It is a known design practice to use a heater core, a radiator-like device, for heating a cabin of a vehicle. Hot coolant from the vehicle's engine may be passed through a winding tube of the core (a heat exchanger between coolant and cabin air.) Fins attached to the core tubes may increase the surface area for heat transfer to air that is forced past them by a fan, for example, to heat the passenger compartment.
Once the engine has warmed up, the coolant may be kept at a generally constant temperature in a known fashion by a closed-loop control system that includes a thermostat. The temperature of air entering the vehicle's cabin may be controlled by a valve that limits the amount of coolant passing through the heater core. Alternatively, the heater core may be blocked off, or partially blocked off, by a valve that directs part or all of the incoming air around the heater core. Some systems allow a driver to control the valve directly by means of, for example, a rotary knob or lever. Other systems may use electronics to control the valve.
Vehicles with dual climate control functions (allowing a driver and passenger to each set a different temperature) may use a heater core split in two, where different amounts of coolant may flow through the heater core on either side of the split to obtain the desired heating.
Because the heater core cools the heated coolant from the engine, it may act as a secondary radiator for the engine. If the primary radiator is working improperly, an operator may turn the heat on in the passenger cabin, resulting in some cooling of the engine.